Capillary reference half-cell

ABSTRACT

The present invention is a reference half-cell electrode wherein intermingling of test fluid with reference fluid does not affect the performance of the reference half-cell over a long time. This intermingling reference half-cell may be used as a single or double junction submersible or surface reference electrode. The intermingling reference half-cell relies on a capillary tube having a first end open to reference fluid and a second end open to test fluid wherein the small diameter of the capillary tube limits free motion of fluid within the capillary to diffusion. The electrode is placed near the first end of the capillary in contact with the reference fluid. The method of operation of the present invention begins with filling the capillary tube with a reference solution. After closing the first end of the capillary, the capillary tube may be fully submerged or partially submerged with the second open end inserted into test fluid. Since the electrode is placed near the first end of the capillary, and since the test fluid may intermingle with the reference fluid through the second open end only by diffusion, this intermingling capillary reference half-cell provides a stable voltage potential for long time periods.

This invention was made with Government support under ContractDE-AC06-76RLO 1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus forproviding a reference half-cell or more specifically to a capillary typereference half-cell having a liquid reference solution constrained fromfree flow volume discharge.

BACKGROUND OF THE INVENTION

Investigation of the chemistry of natural waters including but notlimited to underground aquifers, and above ground lakes is usuallyaccomplished by taking water samples and analyzing the samples in alaboratory. Many chemical species are detected and measured for variouspurposes. For example, waste water discharge may be monitored for thepresence of hazardous chemicals including but not limited to inorganiccations and anions, for example, sulfur compounds and metal compounds,and organic compounds. Another application is the detection andquantification of chemical tracers which are used for determining flowpatterns. Chemical tracers include but are not limited to bromides,chlorides, sulfides, and pH. In addition, chemical analyses areroutinely performed for chemical process flow stream monitoring ofprocesses including but not limited to winemaking, electroplating,hydrometallurgy, papermaking, chemical manufacture, and many otherindustrial processes.

Analysis for determining the presence and amount of particular chemicalspecies may be carried out using an electrochemical cell wherein thevoltage potential is related to a difference in chemical concentrationbetween a known reference solution and the unknown sample test solution.The classic electrochemical cell is two beakers having solutions ofdiffering concentrations or compositions with a salt bridge in contactwith both solutions and electrodes in each solution which are connectedto a voltmeter. The classical electrochemical cell has the attributes of(1) physical isolation of the two solutions, and (2) electricalcommunication (via a salt bridge) between the two solutions, and isuseable in a laboratory setting, but is inconvenient for fieldapplications, especially submerged in-situ measurement applications.

Various methods and devices relying on the physical principles of theclassic electrochemical cell have been used in field applications. Infield applications, the beakers of the classic cell are replaced with asensing half-cell and a reference half-cell which are placed in a testsolution and connected by a voltmeter. Reference half-cells for fieldapplications have the same two attributes of solution isolation withelectrical communication as the classical electrochemical cell.Isolation of the solutions is fundamentally necessary becauseintermingling of solutions would change the electrochemical potential.Reference half-cells for field applications, grouped according to howsolution intermingling is prevented, tend to be of two main types;non-flowing and flowing, wherein the reference solution either flowsfrom the half-cell or it remains within the half-cell.

Electrochemical reference half-cells of the non-flowing type include theliquid filled classical cell and a gel filled submersible cell, forexample, a Model 13-620-259 gel-filled calomel reference half-cell,manufactured by Fisher Scientific Company, Pittsburgh, Pa. In gel-filledcells, an amount of reference solution is placed within a vessel andremains within the vessel, hence the solution is non-flowing. In theclassic cell, the connection between the non-flowing reference solutionand the test solution is a salt bridge, and in the submersible cell, itis a virtually non-porous solid which closes one end of the vesselcontaining the reference solution. Hence, as in the classical cell, thegel-filled cell reference solution gel is physically prevented fromintermingling with test solution yet is in electrical contact throughthe virtually non-porous solid. The virtually non-porous solid forms anelectrical junction. Ideally, it is desirable to minimize the effect ofsuch a junction on the operation of the half-cell. The effect of thejunction is minimized by making the virtually non-porous solid as shortor thin as practical. The classic cell cannot be submerged since theopen beakers would not prevent entry of fluid or liquid in which thecell is submerged, thereby spoiling the concentration of the referencesolution liquid within the cell. The submersible cell is completelyfilled with an incompressible gel so that when the cell is submerged,fluid or liquid cannot enter the cell. However, the virtually non-poroussolid causes variable junction potential and an electrically noisysignal thereby limiting the accuracy of measurements made using thisdevice.

Electrochemical reference half-cells of the flowing type include, forexample, a Model 13-620-216 Ag/AgCl reference half-cell, manufactured byFisher Scientific Company, Pittsburgh, Pa. Half-cells of this typerequire reference solution to flow or leak into the test solution. Theflow is controlled by a porous or fritted opening that allows referencefluid to flow from the vessel. As in the gel-filled half-cell, the fritcreates an electrical junction. The behavior of the electrical junctionis stabilized by allowing the reference solution fluid to flow from thevessel into the test solution fluid. Therefore, the flowing half-cellmaintains a more constant voltage potential compared to the non-flowinggel-filled half-cell. However, with a flowing half-cell, one has atradeoff between making measurements only during the time (often limitedto several hours) that there is sufficient reference solution fluid inthe half-cell, or periodically adding sufficient reference solutionfluid to allow longer term measurements. Moreover, the flowing half-cellis not submersible because the flow would cease or reverse therebydiluting the reference solution fluid with test solution fluid withinthe vessel.

A flowing, capillary type half-cell reduces the amount of referencesolution liquid needed to provide a stable, constant voltage potential,as compared to a flowing non-capillary half-cell. A flowing, capillarytype, for example, a Hach One model 44250 single junction referencehalf-cell, manufactured by Hach Chemical Co., Loveland, Colo., isfundamentally different from the non-capillary half-cells in that theend of the capillary is open rather than closed with a non-porousmaterial or frit. Nevertheless, an electrical junction is formed by theinterface between the two fluids, specifically liquids. This liquidjunction is ideal because there is no plug material thereby producing avery stable signal. An electrode is mounted within the capillary andnear the liquid junction close to the open end of the capillary. Thereference solution liquid is in a syringe connected to the capillarytube. The capillary reduces the volume of reference solution liquidneeded to flow into the test solution liquid and thereby maintain astable voltage potential. In operation, the syringe is depressed a smallamount to discharge reference solution liquid from the open end of thecapillary prior to making a measurement.

Since the capillary is open, intermingling of the test solution liquidand the reference solution liquid within the capillary will eventuallychange the concentration of reference solution liquid at the electrodeand require an additional discharge of reference fluid. Although thiscapillary reference half-cell has the advantage of stability, and it isconvenient for benchtop measurements because the reference solutionliquid is easily replenished, the ease of replenishment does not permitsubmerged operation and the proximity of the electrode to the open enddoes not permit prolonged operation because it requires frequent flow ofreference solution liquid.

There is yet another reference half-cell described in U.S. Pat. No.3,705,089 to Grubb that is gel-filled but open ended. However, the gelthat is in direct contact with test solution liquid changes as a resultof that contact thereby affecting the electrical potential of thehalf-cell. Grubb identifies the need to "renew" the gel/test solutionliquid junction. In this case, the liquid junction is formed by aninterface between the reference solution gel and a test solution liquidwherein the interface is distinct and the reference solution, being agel, does not flow through the tube for operation of the half-cell.Grubb does not describe how the interface or junction degrades, butclearly indicates that renewal is necessary. Renewal of the junction isaccomplished by cutting off a small segment of the gel-filled tube atits open end. Since cutting and removing material is, in general terms,a bulk volume discharge, this half-cell may be considered of the"flowing" type. A disadvantage of this half-cell is the need to renewthe junction by cutting thereby limiting both the time betweenmeasurements and the remoteness of measurements.

All of the reference half-cells discussed and described above are of thesingle junction type. In some applications, it is desirable to have adouble junction reference half-cell. The flowing type cell can be usedas a double junction cell by placing a first vessel having a frittedopening within a second vessel having a fritted or ground glass opening.The solution fluid in the second vessel is different from the referencesolution fluid in the first vessel. The main advantage of a doublejunction half-cell is that the solution fluid in the outer vesselphysically and chemically isolates the reference solution from the testsolution fluid while maintaining electrical communication between thetwo solution fluids.

It is apparent from the foregoing discussion that prior to the presentinvention, there was no known apparatus or method providing a half-cellthat did not require renewal of the liquid junction for stableelectrochemical measurements. Further, before the present invention,there was no known submersible capillary half-cell, nor was there adouble junction submersible capillary half-cell. It would beadvantageous to have a double junction submersible reference half-cellfor detecting and measuring concentrations of chemical species. Thoseskilled in the art would further find advantages in a referencehalf-cell either single or double junction that did not requirereplenishment of a reference solution fluid yet provided a stablevoltage potential over a long time period of at least several weeks. Thepresent invention further provides ability to make in-situ chemicalmeasurements in real time at a substantially lower cost than bylaboratory analysis of field samples.

SUMMARY OF THE INVENTION

The present invention is a capillary reference half-cell allowingintermingling of test solution liquid within reference solution liquidwithin the capillary. The reference half-cell of the present inventionmay be used as a single or double junction submersible or surfacereference half-cell. The intermingling reference half-cell relies on acapillary tube having a first end open to reference fluid and a secondend open to test solution liquid wherein the small diameter of thecapillary tube limits free motion of fluid within the capillary todiffusion. An electrode is placed near the first end of the capillary incontact with the reference solution liquid. The length of the capillarydetermines the service life of the intermingling reference half-cell.

The method of operation of the present invention begins with filling thecapillary with a reference solution liquid. After closing the first endof the capillary, the capillary may be fully submerged or partiallysubmerged with the second open end inserted into test solution liquid.Since the electrode is placed near the first end of the capillary, andsince the test solution liquid may enter the second open end only bydiffusion, this intermingling reference half-cell provides a stablevoltage potential for extended time periods, up to several weeks. Thesubject matter of the present invention is particularly pointed out anddistinctly claimed in the concluding portion of this specification.However, both the organization and method of operation, together withfurther advantages and objects thereof, may best be understood byreference to the following description taken in connection withaccompanying drawings wherein like reference characters refer to likeelements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an intermingling capillary half-cell;

FIG. 2 is a sectional view of an intermingling capillary half-cell witha reference fluid reservoir;

FIG. 2a is a cross section of a further embodiment of an interminglingcapillary half-cell;

FIG. 2b is a cross section of modified threads;

FIG. 2c is a cross section of a further modification of threads;

FIG. 2d is an intermingling capillary half-cell with a signalconditioning circuit;

FIG. 2e is a combination electrode incorporating an interminglingcapillary half-cell;

FIG. 3 is a plot of voltage potential versus analyte activity for theexperiment of Example 1;

FIG. 4a is a plot of cell potential versus time at a depth of 1.5 feetfor the experiment of Example 2;

FIG. 4b is a plot of cell potential versus time at a depth of 19.5 feetfor the experiment of Example 2; and

FIG. 4c is a plot of cell potential versus time at a depth of 31.5 feetfor the experiment of Example 2;

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

An embodiment of an intermingling capillary 0 reference half-cell (1) isshown in FIG. 1. The main component of the reference half-cell (1) is acapillary tube (2) having a first end (4) and a second end (6).Reference fluid (8) may be introduced into the capillary tube (2) byinjecting the reference solution liquid (8) under positive pressure intoone end or by drawing reference solution liquid (8) under negativepressure. The first end (4), is closed after receipt of the referencesolution liquid (8), while the second end (6) remains open. In use, atleast the second end (6)is immersed in a test solution liquid (10)whereby the reference solution liquid (8) is in direct open contact withthe test solution liquid (10) while the reference solution liquid (8)substantially remains within the capillary tube (2).

The second component of the reference half-cell (1) is an electrodeelement(12) placed near the first end (4) of the capillary tube (2) andin contactwith reference solution liquid (8). A wire (14) provideselectrical contactbetween the electrode element (12) and a voltmeter(not shown).

The use of a capillary tube (2) having a closed end (4), and filled withsubstantially incompressible reference solution liquid (8) constrainsthe reference solution liquid (8) from free flow volume discharge underthe influence of atmospheric pressure or hydraulic pressure of the testsolution liquid (10). Therefore, the reference solution liquid (8)remainswithin the capillary tube (2) and remains pure except as it isdiluted by either diffusion between the reference solution liquid (8)and the test solution liquid (10) at the second end (6), or by flowinduced by thermal expansion or contraction of the capillary tube (2)when the coefficient ofthermal expansion of the capillary tube (2) isdifferent from the coefficient of thermal expansion of the referencesolution liquid (8), andthe reference half-cell is at a temperaturedifferent from the test solution liquid (10) into which it is immersed.Therefore, the reference electrode element (12) "sees" pure referencesolution liquid (8) until test solution liquid (10) is drawn bydiffusion or thermally induced flow near the electrode element (12). Theamount of time that the reference solution liquid (8) remains puredepends on the concentration differences of the chemical species in theliquids and depends on the distance betweenthe second end (6) of thecapillary tube (2) and the electrode element (12). Hence, the time inservice of the present invention depends upon thelength of the capillarytube. The capillary tube (2) may be of any materialchemically compatiblewith the reference solution liquid (8) and the test solution liquid(10). The preferred material is plastic for ease of construction andhandling including coiling the capillary tube (2). It is preferred thatthe open end (6) of the capillary tube (2) is turned upwardto avoid thepossibility of trapping an air bubble in the open end (6) and opencircuiting the reference half-cell (1).

A second embodiment is shown in FIG. 2 wherein a reference fluidreservoir (16) is sealably attached to the capillary tube (2) near thefirst end (4). The reference solution liquid reservoir may have a septum(18) for admitting reference solution liquid (8) with a syringe (20).The reservoirmay be deformable like a squeeze bulb or plunger, orattached to a squeeze bulb for drawing reference solution liquid intothe reservoir (16) and capillary tube (2). Reference solution liquid (8)may be admitted by any of several means including but not limited to (1)submersion, (2) osmosis across a semi-permeable membrane, (3) vacuumpump or vacuum bottle and valve arrangement, and (4) positive pressurepump and valve arrangement.

Additional features may be added for convenience of operation. An outerhousing (22) may be placed around a portion of the reference half-cell(1)creating an annular space (24) that may be filled with material (26)havinga density greater than that of water as an aid to submerging thereference half-cell (1).

An enlarged section (28) may be added to provide increased volumebetween the open end (6) of the capillary tube (2) and the electrodeelement (12).The increased volume mitigates thermally induced flow ofthe reference solution (8). The enlarged section may be deformable foradmitting or expelling liquid from the capillary.

A port (30) and cap (32) may be provided separately or in combinationwith the enlarged section (28) for filling the capillary tube (2) withreference solution liquid (8). The port (30) may be any tee or fluidtightattachment but is preferably a female hypodermic needle fitting andthe cap(32) may be any closure that is sealably compatible with the port(30) but is preferably a male hypodermic needle fitting.

Either embodiment shown in FIGS. 1 or 2 may be used as a double junctionreference half-cell. A double junction may be made by immersing thesecondend (6) of the capillary in a non-interfering salt solution liquidthen drawing sufficient salt solution liquid into the capillary tube todisplace about half of the reference solution liquid. Alternatively, theport (30) may be used to introduce a non-interfering solution liquid ina portion of the capillary tube (2) between the port (30) and the openend (6) of the capillary tube (2) for making a double junctionhalf-cell. The electric potential provided by this reference half-cellis stable until diffusion causes the reference solution liquid (8) nearthe electrode (12)to be diluted by either the non-interfering solutionliquid or the test solution.

Various combinations of reference solution liquid (8) and electrodeelement(12) may be used. Examples include but are not limited to (i) areference solution liquid (8) of 4 molar potassium chloride saturatedwith silver chloride in combination with an electrode element (12) ofsilver chloride electrolytically deposited on silver, and (ii) areference solution liquid(8) of saturated calomel with an electrodeelement (12) of calomel electrolytically deposited on silver.

Further, as shown in FIG. 2a, the capillary may be formed by modifying ascrew type thread (40) thereby providing a helical capillary. An outerhousing (42), having a first end (4) that is closed and a second end (6)that is open, is provided having internal threads (44). A rod (46)having outer threads (48) is threadably engaged into outer housing (42).With reference to FIG. 2b, one sees that the outer threads (48) havebeen modified to have a helical flat surface (49) thereby creating ahelical capillary opening (50) between the outer threads (48) and theinternal threads (44). The rod (46) permits passage of wire (14) to thereference electrode element (12), preferably via a hole (54). The hole(54) is preferably sealed with a sealing compound.

Reference solution liquid is placed within the outer housing (42) andthe rod (46) threadably engaged thereby obtaining reference solutionliquid ina reservoir volume (52) and in the helical capillary (50). Thereference electrode element (12) is positioned within the reservoirvolume (52) nearthe first end (4). This embodiment may be modified byaddition of a port (30) and a sealable cap (32) to provide a doublejunction half-cell reference electrode as illustrated in FIG. 2d.

It will be apparent that many modifications are possible wherein thecapillary is formed by other mechanical interfaces between mating parts.For example, the inner thread (44) may be modified in addition to orinstead of the outer thread (48). The thread may be modified in otherwaysas illustrated in FIG. 2c wherein the helical capillary (50) isdefined by a helical groove (60).

A yet further embodiment is the use of a syringe wherein the outerhousing is not threaded but an inner slidable syringe plunger has ahelical groovethereon forming a capillary between the helical groove andthe smooth wall of the outer housing.

It is further recognized that the invention is not limited to circularcross section geometry. A slidable syringe plunger may be of any crosssection. A threadably engaged capillary may be of any cross sectionprovided the outer housing may be split and re-sealable.

It will be further appreciated that, while convenient, the invention isnotlimited to helical shaped capillary. A serpentine groove on a flatsurface placed adjacent to a second flat surface defines a capillaryoperable according to the present invention.

There are several advantages of a capillary formed by mechanicalinterface of mating parts including ease of cleaning the walls of thecapillary. Thethreaded rod and housing is especially advantageousbecause handling a longcapillary tube is avoided.

It will be appreciated by those skilled in the art of electrochemicalcellsthat the cells typically have high internal electrical resistancethereby yielding a high impedance or "weak" electrical signal. There aremany waysto measure weak electrical signals, and it is generallyrecognized that a high impedance meter, eg. voltmeter, is needed.

Even with a high impedance meter, there are applications wherein a weaksignal is adversely affected. For example, when an analog signal istransmitted over a long distance, eg. 300 ft, the internal electricalresistance of the long wire and outside signals such as radio signalscan interfere with the measurement signal. There are many ways ofconditioningthe signal to overcome these problems including but notlimited to signal digitizing and signal amplification. In the presentinvention, the signal current is amplified with an operational amplifiervoltage-follower circuit. The circuit may be included with theintermingling capillary electrochemical half-cell as shown in FIG. 2d.The circuit (62) is sealed within an housing extension (64) of the outerhousing (42). Electrical power to the circuit may be provided by anymeans, and the electrical power source may be remote- or proximate tothe intermingling capillary electrochemical half-cell. It is preferredthat the electrical power source be remote from the half-cell, and nearthe meter. This arrangement,of course, requires two additionalelectrical leads between the intermingling capillary electrochemicalhalf-cell and the meter. It will be appreciated by those skilled in theart of instrument signal conditioning that an advantage of anoperational amplifier circuit is thatits output signal is compatiblewith a wide variety of signal reporting instruments including but notlimited to data loggers, strip chart recorders, and lower impedancevoltmeters.

A yet further embodiment is shown in FIG. 2e. This embodiment is a"combination electrode" comprised of a sensing half-cell (70),intermingling capillary reference half-cell (1), and signal conditioningcircuit (62). The sensing half-cell (70) comprises a sensing electrode(72) and wire (73) and a test solution liquid (10). The sensingelectrode (72) may be any ion selective electrode, for example a bromideion selective electrode that is silver having a coating of silverbromide. Thesensing electrode (72) may be of any convenient shape andmay be placed in any location on the combination electrode. The wire(73) may be placed internally or externally, but is preferably internalproximate to the wire(14).

For submersible applications, the combination electrode must sink. Aweight(74) may be added wherein the weight has a density greater thanthe liquid,usually water, into which the combination electrode issubmerged. The weight may be of any such material including but notlimited to shot, and solid stainless steel.

EXAMPLE 1

An experiment was conducted using an intermingling capillary referencehalf-cell with a double junction constructed in accordance with thepresent specification and drawings. The purpose of the experiment was tomeasure the concentration of a bromide tracer test solution liquid usingafirst reference half-cell together with a second sensing half-cell ofsilver bromide on silver, with both half-cells connected to a voltmeter.

The intermingling reference half-cell utilizes a capillary tube of about60cm in length and having an internal diameter of about 0.03 cm. Thecapillary tube was first filled with a reference solution liquid of 4molar potassium chloride saturated with silver chloride by injecting thereference solution liquid through the septum with a hypodermic needleand syringe. Next, a non-interfering solution liquid of 10 percentpotassium nitrate was added through the port into the lower half of thecapillary tube, thereby establishing the referencesolution/non-interfering solutionliquid interface within the enlargedsection of the capillary tube. An electrode element of silver chlorideon silver was placed within the reference solution liquid reservoir witha surface of the electrode element in contact with the referencesolution liquid. The non-interferingsolution liquid is needed to preventthe chloride of the reference solutionliquid from entering the testsolution liquid, to prevent any reaction between the bromide of the testsolution and the silver of the reference solution liquid.

A 4 liter graduated cylinder was filled with tap water and stirred forabout a day to insure a constant temperature and composition throughoutthe volume of water. Water temperature stabilized at about 23° C. Wateranalysis showed that the water contained calcium bicarbonate and calciumsulfate and had an ionic strength of about 0.0044 molar. The referenceand sensing half-cells were immersed into the water and allowed tostabilize for about 21/2 hours.

Potassium bromide was added to the water over a 3 hour period inincrementsso that the bromide concentration ranged from 0.17 to 50.0milligrams per liter. For each increment, the activity coefficient andthe activity of the bromide were calculated based upon the ionicstrength of the water andthe additional ionic strength of the potassiumbromide according to Nernst-Peters equation provided in Garrels, R. M.,and Christ, C. L., Solutions, Minerals, and Equilibria, Harper & Row,NY, 1965.

Results are shown in FIG. 3. The electropotential as measured inmillivoltsis linear with the logarithm of activity of the bromide forbromide concentrations between 1 and 50 milligrams per liter. Thenon-linearity atvery low activities is characteristic of ion-selectivehalf-cells. In this experiment, the non-linearity is caused by theslight solubility of the silver bromide of the sensing half-cellelectrode. The upper left data point (40) is the electropotential of thewater before addition of potassium bromide plotted against the activityof bromide calculated from the solubility product of silver bromide aslisted in the Handbook of Chemistry, N. H. Lange, Handbook Publishers,Inc., Sandusky, Ohio, 1956.

The graduated cylinder was then covered to prevent evaporation andstirringmaintained for about 19 hours. Total drift of theelectropotential readingswas -3.7 millivolt representing an apparent 16percent increase of bromide activity. The electrochemical potential wasthen stable at 31.6 millivolts. At about the 20th hour, 0.32 grams ofsodium sulfite powder was added to the water. The electropotentialincreased by 4 millivolts, but within 2 hours stabilized at 33.6millivolts, which was the expected value after the salt addition.

During the two hours after the salt addition, electropotential drift was-3.5 millivolts. During the next 66 hours (total elapsed time about 116hours), total drift was about -0.5 millivolts.

At about the 117th hour, sodium sulfide was added in two increments totestthe response of the second sensing electrode to a sulfide chemicalinterference. The first addition was 0.066 milligrams per liter and thesecond was 0.325 milligrams per liter. The electropotential differencebetween these two increments was 2.1 millivolts. Analysis using theNernst-Peters equation (Garrels and Christ, 1965) shows that theresponse of the sensing half-cell to sulfide was 5 percent of thetheoretical Nernst slope. It was observed that the bromide electrode wasvisibly darkened by a coating of silver sulfide on the silver bromide.

Two hours later (at about the 119th hour), the bromide concentration wasincreased from 50 to 82 milligrams per liter and the bromide activitycalculated. The measured change in electropotential due to bromideaddition was 11.9 millivolts, which is again 95 percent of thetheoreticalNernst slope. Hence, the change in electropotential due to anincrease in bromide concentration was unaffected by either the silversulfide on the bromide electrode or the bisulfide in the test solutionliquid.

For the next 18 days, (total elapsed time of about 23 days) theelectrodes and test solution liquid were undisturbed, and total drift ofelectropotential was -0.4 millivolts.

At about the 23rd day, 10 milliliters of potassium bromide were added tothe test solution liquid. Analysis showed that the electropotentialresponse was still 95 percent of the theoretical Nernst slope. Theelectrodes and test solution liquid were left undisturbed for anadditional 6 days during which time the total electropotential drift was-0.2 millivolt.

Over the total elapsed time of about 29 days, both the first referencehalf-cell and the second sensing half-cell demonstrated reliablestabilitywith very little drift. The only significant undesirable driftin electropotential occurred during the first day of the experiment. Themagnitude of this undesirable drift is typical of electrodes that areput into service after having been stored in a dry condition.

EXAMPLE 2

A second experiment using the present invention was carried out underfieldconditions in a water well using a first non-flowing capillaryreference half-cell With a double junction, together with a secondsensing half-cellin accordance with Example 1.

Two 100 foot lengths of insulated electrical wire were connected to bothhalf-cells and the connections were made water proof with a commercialsealant. The opposite ends of the wires were connected to a highimpedancevoltmeter. The half-cells were immersed in water for about 17hours prior to the experiment. The purpose of the immersion was to avoidexcessive initial electropotential drift upon immersion of thehalf-cells into the well water.

After 17 hours, the half-cells were calibrated using solution liquids ofnatural ground water from the test well with known concentrations ofreagent grade lithium bromide. Results of calibration showed that a 10mg/L bromide solution liquid yielded a voltmeter reading of 62.1millivolts, and that the bromide-sensing half-cell was responding at56.2 millivolts per decade of decreasing bromide concentration, which is95 percent of the theoretical Nernst slope.

Bromide was distributed within the vertical water column of the well asevenly as practical, and based on depth and diameter of the well, theinitial average concentration of bromide was expected to be 202 mg/L.The half-cells assembly was lowered into the well and initial readingswere taken at twelve 3 foot intervals along the vertical water column.Subsequent sets of readings were taken periodically.

It was expected that the bromide concentration would decrease withsubsequent sets of readings as water in the well was refreshed by thegroundwater, resulting in an increase in the voltage potential of thehalf-cells. It was further expected that the millivolt readingscorresponding to the logarithm of the bromide activity would increaselinearly with time. Indeed, as illustrated in FIGS. 4a, 4b, and 4c,graphsof millivolt readings with time for depths of 1.5 feet, 19.5 feet,and 31.5feet respectively are increasingly linear within experimentalerror.

Extrapolation to time t=0.0 minutes, results in initial bromideconcentrations at various depths ranging from 186 mg/L to 248 mg/L asgiven in Table 1. The initial bromide concentrations in. Table 1representa 3 foot vertical segment of the well except for the deepestinterval whichrepresents a 4 foot segment. A weighted average of theinitial bromide concentrations of Table 1 yields an estimate of 206mg/L, which is very close to the expected value of 202 mg/L.

                  TABLE 1                                                         ______________________________________                                        Initial Bromide Concentrations                                                Bromide concentration (mg/L)                                                  Depth at time = 0.0                                                           ______________________________________                                         1.5 219                                                                       4.5 202                                                                       7.5 194                                                                      10.5 186                                                                      13.5 194                                                                      16.5 186                                                                      19.5 186                                                                      22.5 179                                                                      25.5 194                                                                      28.5 194                                                                      31.5 202                                                                      34.5 248                                                                      ______________________________________                                    

The close agreement of the measured and expected initial bromideconcentrations, together with the linear nature of the graphsdemonstrate accurate and stable performance of the half-cells. Further,the half-cellswere unaffected by the variation in hydraulic pressurebetween 1.5 and 34.5feet of water.

While a preferred embodiment of the present invention has been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinventionin its broader aspects. The appended claims are thereforeintended to coverall such changes and modifications as fall within thetrue spirit and scopeof the invention.

I claim:
 1. A capillary reference half-cell for immersion into a testsolution liquid, comprising:(a) a capillary having a length furtherhaving a portion of an amount of reference solution liquid therewithin,(b) said capillary constraining said portion of reference solutionliquid from free flow volume discharge and said capillary having a firstand second end, said first end further having a closure, said second endopen, thereby forming at least one liquid junction between saidreference solution liquid and said test solution liquid, and permittingdiffusion between said reference solution liquid and said test solutionliquid, and (c) an electrode element placed in contact with saidreference solution liquid nearer said first end than to said second endwherein a voltage potential is related to a chemical concentration ofsaid reference solution liquid, said voltage potential is constant whilediffusion occurs until the diffusion dilutes said reference solutionliquid near said electrode element so that a time in service dependsupon the length of the capillary.
 2. A reference half-cell as recited inclaim 1, wherein said reference solution liquid is potassium chloridesaturated with silver chloride and said electrode element is silverchloride on silver.
 3. A reference half-cell as recited in claim 1,wherein said reference solution liquid is saturated calomel and saidelectrode element is calomel on silver.
 4. A reference half-cell asrecited in claim 1, wherein a deformable fluid reservoir is placed onsaid first end for use in admitting reference solution liquid into saidcapillary.
 5. A reference half-cell as recited in claim 1, wherein saidclosure includes a fluid reservoir having a septum, said reservoirplaced on said first end for admitting reference solution liquid intosaid capillary.
 6. A reference half-cell as recited in claim 5, whereinsaid electrode is mounted in said fluid reservoir.
 7. A referencehalf-cell as recited in claim 1, wherein said closure includes a fluidreservoir having a septum, said reservoir placed on said first end foruse with a syringe for admitting reference solution fluid into saidcapillary.
 8. A reference half-cell as recited in claim 7, wherein saidelectrode is mounted in said reservoir.
 9. A reference half-cell asrecited in claim 1, wherein said capillary is a tube and furthercomprises:an enlarged section between said first end and said second endproviding increased volume of said reference solution liquid.
 10. Areference half-cell as recited in claim 1, wherein said capillary is atube.
 11. A reference half-cell as recited in claim 1, wherein saidcapillary is formed by a mechanical interface between mating parts. 12.A reference half-cell as recited in claim 11, wherein said mating partsare a housing threadably engaged with a rod.
 13. A reference half-cellas recited in claim 12, further comprising a signal conditioning circuitmounted in a housing extension.
 14. A combination electrode,comprising:(a) a capillary having a length further having a portion ofan amount of reference solution liquid therewithin; (b) said capillaryconstraining said portion of said reference solution liquid from freeflow volume discharge and said capillary having a first and second end,said first end further having a closure, said second end open, therebyforming at least one liquid junction between said reference solutionliquid and said test solution liquid, and permitting diffusion betweensaid reference solution liquid and said test solution liquid; (c) anelectrode element placed in contact with said reference solution liquidnearer said first end than to said second end wherein a voltagepotential is related to a chemical concentration of said referencesolution liquid, said voltage potential is constant while diffusionoccurs until the diffusion dilutes said reference solution liquid nearsaid electrode element so that time in service depends upon the lengthof the capillary; and (d) a sensing electrode mounted exterior to saidcapillary and in contact with said test solution liquid, thereby forminga sensing half-cell.
 15. A combination electrode as recited in claim 14,further comprising a signal conditioning circuit mounted in a housingextension.
 16. A method of using a capillary reference half-cell,comprising the steps of:(a) filling a capillary with a portion of areference solution liquid, said capillary having a first and second end,said first end closed, said second end open, (b) placing an electrodenearer said first end than to said second end and in contact with saidreference solution liquid, (c) inserting said second end into a testsolution liquid together with a second sensing half-cell for obtainingelectrochemical measurements, and (d) preventing free flow volumedischarge of said reference solution liquid into said test solutionliquid.
 17. A method as recited in claim 16, furthercomprising:displacing an amount of said reference solution liquid withnon-interfering liquid solution between from said second end to a lengthof said capillary and forming a double junction reference half-cell. 18.The method as recited in claim 16 wherein said capillary is initiallyfilled with a first reference solution liquid, then a secondnon-interfering liquid solution is drawn into the second end with aninterface between said first and second solutions between said first andsecond ends of said capillary, for providing a double junction referencehalf-cell.